1. Field of the Invention
This invention relates to an impact testing device and, more particularly, to an apparatus for determining the impact strength of a racket, clamped to a pivot arm, which moves into contact with a fixed, variably positionable, impact plate.
2. Description of the Background Art
There are a series of standard destructive tests within the tennis industry that have been established to evaluate the durability of a composite tennis racket. Each manufacturer may specify different criteria or levels for each test, but will typically use each test in the qualification and inspection of production rackets. These test will normally involve a fatigue test, a horizontal and vertical crush test, and a drop test. The test level required for a racket will be set based on the designer's experience rather than basing it on some correlation between the test and an equivalent force load level experienced during play.
The only impact type test that is typically performed on tennis rackets is called a head drop test. The head drop test involves dropping a racket strung at a specified string tension from a given height on the head of a racket. A standard set-up attaches approximately a 400 gram clamp to the butt of a racket, raises the racket via a string attached to the clamp to a specified height as measured from the lowest point of the racket, and drops the racket on its head. The racket is then inspected for cracks. The requirement for passage of a racket may involve a single or multiple drops from a predetermined height without cracks in the frame. Most likely because impact with a rigid object, i.e., the court, is rather uncommon in tennis, this test seems to be able to adequately predict whether a tennis racket will fail due to impact with the court.
In a racquetball or squash game, impact between the racket and the court, whether floor, side walls or back wall, are much more common and must be taken into account in devising a racket that meets the durability requirements of each game. Impact between the wall and the racket may occur over the entire top half of the head of a racket. The head drop test alone does not sufficiently predict the relative impact strength of a racket for these games. This is probably because the impact point at the top of the head of a racket (12 o'clock position) and the applied load decreases rapidly as you move away from this point. A new test was needed to test the relative impact strength of a racket over the entire top half of the racket.
The present design is similar to an Izod impact test, in that it involves measuring the maximum angle at which a swing arm can be raised and released freely without breaking a particular object, in this case a racket. In the Izod test the test specimen is secured on a fixed support at the bottom of the arc of the swing arm. In the racket impact fixture of the present invention, the racket is attached to the swing arm and impacts against a rigid plate. The plate can be oriented at different angles to vary the location of the impact between the racket and the plate. Through trial and error it was determined that clamping of both the racket handle and the string bed of the racket provided the best correlation to actual impact strength of a racket.
The strength of a particular racket can most readily be established by comparing the angle at which it fails to that of another racket. The relationship between the angle of that swing arm was raised and the energy involved in the subsequent impact is not linear. A calculation of the energy involved in the impact can also be made so that a more quantitative comparison can be made. The calculation involves determining the total weight of the swing arm including the racket (M), finding the location of the center of gravity (Cg), determining the radial distance (R) between the pivot point and the Cg, and calculating the location or height (H) of the Cg at the point form which the swing arm will be released. Assuming no friction in the bearing at the pivot point, then the impact energy (E) will be equal to the potential energy of the swing arm and racket before it is released (E=MGH). The height of the Cg is related to the angle of the swing arm (theta) by the formula H=R+R.times.sin (theta) .times.tan (theta) for angles less than 90 degrees and H=R+R.times.tan (theta- 90) for angles greater than 90 degrees.
For simplicity, impact testing has been limited to impacts with the impact plate at a vertical position and at a 45 degree angle. All rackets tested to this point have failed at lower energies (angles) with the plate at the 45 degree angle, than with the plate at the 90 degree angle. Therefore, the majority of testing has been performed with the plate in this position. Impact of the racket with the plate occurs roughly at the 10 or 2 o'clock positions with the plate fixed at 45 degrees. This is also the most common position in which rackets fail during actual play. Thus far rackets with constructions that fail at higher impact energies in the impact fixture also are failing less in actual play.
Although many types of impact testing devices are known and are in wide use today throughout various industries, none provides the benefits of the present invention. Typical examples of impact testing are described in the patent literature. Note for example, U.S. Pat. Nos. 1,984,904 to Warshaw; 2,022,666 to Haskell; 2,388,246 to Berger; 2,476,297 to Harris; and 3,083,586 to Carter. Each of these patents relate to an impact testing device wherein the object to be tested is fixedly positioned and a moving arm moves into contact therewith at a predetermined speed.
Note is also taken of U.S. Pat. No. 3,566,668 to Browning which relates to an impact testing of an article on a swing arm but has no variable positionable fixed surface to be contacted. Lastly, U.S. Pat. No. 3,885,842 to Imabori describes an impact testing device where a swing arm supports a golf club for testing the dynamic performance of the club.
No prior impact testing device, however, has the capability as does the present invention by utilizing 3T25 swing arm to pilot a racket into contact with fixed, variably positioned plate, for determining the impact strength of the racket.
Accordingly, it is an object of the present invention to provide an apparatus to test for the impact strength of a game racket comprising a base plate having an upper surface; a support post extending upwardly from the upper surface of the base plate; a pivot arm having a first end and a second end with clamping components adjacent to the first end adapted to releasably secure the racket to be tested with at least a portion thereof extending beyond the first end; a pivot pin rotatably coupling the second end of the pivot arm to the support post adjacent to its upper end; a pointer coupled to the second end of the pivot arm; an indicator plate with indicia thereon cooperable with the pointer to indicate the angle of the pointer, pivot arm and racket supported thereon with respect to the vertical; and an impact place secured to the upper surface of the base plate, the impact plate having a striking surface with a central extent at a location to be in point contact with the frame of a racket to be tested when supported by the swing arm when in a vertical orientation whereby the swing arm and racket may be pivoted away from the impact plate and released to determine the impact strength of the racket and frame.
It is a further object of the present invention to swing an object to be tested into contact with a fixed surface for determining the impact strength of the object.
It is a further object of the present invention to vary the angular position of planar surface into which an object to be tested for impact strength is swung.
It is a further object of the present invention to measure the angle at which an object secured to a swing arm is pivoted in order to determine the maximum angle at which an object can be swung prior to breaking.
These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or by modifying the invention within the scope of the disclosure. Accordingly, other objects and a more comprehensive understanding of the invention may be obtained by referring to the summary of the invention, and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.